'Contact us' attack takes out mail servers

The "contact us" feature on many websites is often insecure and makes it easy to launch denial of service attacks on corporate mail servers, according to UK-based security consultancy SecureTest.

The "contact us" feature is usually a form that allows surfers to submit comments to the people running a website. According to SecureTest, these forms can be used to launch denial of service attacks through endemic security weaknesses that have largely been overlooked.

The significance of the attack varies, depending on whether or not firms host their websites internally. Even sites hosted by third parties can still be vulnerable to denial of service attack through misuse of web-based forms, but the risk is probably worst for firms that cost their own systems.

In cases where firms host their own website, the server would normally sit in the DMZ between external and internal firewalls. The "contact us" form simply creates an email on submission and sends the content to your internal mail server for forwarding on to the relevant internal contact.

Typically, mail filters will treat the web server as an internal mail client, so reduced levels of filtering that may be in place. So if an attacker scripts an attack, battering the form with large volumes of submissions, he can easily cause a mail server to fall over.

Writing scripts to change the content of messages or launching the attack via a network of compromised PCs, therefore changing the source IP address of the submitter, increases the potency of the attack. If the form of features an auto-response the potential load placed on systems by the attack increases all the more, especially if the submitting address is invalid because this generates a bounced reply.

Attacks of this kind could lead to corporate mail systems being overwhelmed in a very short space of time, SecureTest warns.

Managing director Ken Munro said the flaw is overlooked by most organisations, and cited results from the firm's penetration testing work with clients. "When we're penetration testing, we ask clients if they've considered this style of attack. By explicit agreement with the client, with careful consideration to the time of day (or night!) for testing, we conduct a 'contact us' DoS, and in every case we've tried so far, the client's mail server stops responding during the test window," Munro told El Reg.

Fortunately, the attack is relatively easy to defend against.

Firms could rate-limit email traffic from the website, perhaps by using packet shaping techniques, but this risks losing genuine traffic. A better approach is to put a CAPTCHA on the "contact us" form so users are required to enter a short phrase from an image that's not machine readable as part of the form submission process. ®